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        <center>js函数的一些基础知识</center>
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<p>函数是一段可反复调用的代码块。<br>函数能接受输入参数，不同参数返回不同的值。</p>
<h2 id="1-概述"><a href="#1-概述" class="headerlink" title="1. 概述"></a>1. 概述</h2><h3 id="1-1-声明"><a href="#1-1-声明" class="headerlink" title="1.1 声明"></a>1.1 声明</h3><ul>
<li>function命令</li>
<li>函数表达式 </li>
<li>Function构造函数<h3 id="1-2-函数的重复声明"><a href="#1-2-函数的重复声明" class="headerlink" title="1.2 函数的重复声明"></a>1.2 函数的重复声明</h3>一个函数被多次声明，后面覆盖前面。<br>由于函数名提升，前面任何时候都是无效的。<h3 id="1-3-圆括号运算符、return语句和递归"><a href="#1-3-圆括号运算符、return语句和递归" class="headerlink" title="1.3 圆括号运算符、return语句和递归"></a>1.3 圆括号运算符、return语句和递归</h3></li>
<li>圆括号运算符：圆括号中可加入函数参数</li>
<li>return语句</li>
<li>递归（recursion）：函数可以调用自身<h3 id="1-4-第一等公民"><a href="#1-4-第一等公民" class="headerlink" title="1.4 第一等公民"></a>1.4 第一等公民</h3>将函数看作一种值，与其他值地位相同。<h3 id="1-5-函数名的提升"><a href="#1-5-函数名的提升" class="headerlink" title="1.5 函数名的提升"></a>1.5 函数名的提升</h3></li>
<li>用<strong>function命令</strong>声明函数，提升到代码头部。</li>
<li>同时采用<strong>function命令</strong>和<strong>赋值语句</strong>声明一个函数，最后总是采用赋值语句定义。<h3 id="1-6-不能在条件语句中声明函数"><a href="#1-6-不能在条件语句中声明函数" class="headerlink" title="1.6 不能在条件语句中声明函数"></a>1.6 不能在条件语句中声明函数</h3>由于函数名提升，条件语句中声明函数可能是无效的。<br>要达到在条件语句中声明函数的目的，只有使用函数表达式。<h2 id="2-函数的属性和方法"><a href="#2-函数的属性和方法" class="headerlink" title="2. 函数的属性和方法"></a>2. 函数的属性和方法</h2><h3 id="2-1-name属性"><a href="#2-1-name属性" class="headerlink" title="2.1 name属性"></a>2.1 name属性</h3>返回函数的名字。<h3 id="2-2-length属性"><a href="#2-2-length属性" class="headerlink" title="2.2 length属性"></a>2.2 length属性</h3>返回函数预期传入的参数个数。<h3 id="2-3-toStrion"><a href="#2-3-toStrion" class="headerlink" title="2.3 toStrion()"></a>2.3 toStrion()</h3>返回一个字符串，内部是函数的源码（包括函数内部注释）。<h2 id="3-函数的作用域"><a href="#3-函数的作用域" class="headerlink" title="3. 函数的作用域"></a>3. 函数的作用域</h2><h3 id="3-1-定义"><a href="#3-1-定义" class="headerlink" title="3.1 定义"></a>3.1 定义</h3>ES5规范，JS只有两种作用域：</li>
<li>全局作用域：函数外部声明的变量就是全局变量（global variable），可在函数内读取。</li>
<li>函数作用域：函数内部定义的变量就是局部变量（local variable）,外部无法读取。</li>
<li>局部变量会在该作用域内覆盖同名全局变量。</li>
</ul>
<p>ES6新增块级作用域。</p>
<h3 id="3-2-函数内部变量提升"><a href="#3-2-函数内部变量提升" class="headerlink" title="3.2 函数内部变量提升"></a>3.2 函数内部变量提升</h3><p>函数内部var声明的变量，变量声明会被提升到函数体头部。</p>
<h3 id="3-3-函数本身的作用域"><a href="#3-3-函数本身的作用域" class="headerlink" title="3.3 函数本身的作用域"></a>3.3 函数本身的作用域</h3><p>函数执行时的作用域是定义时的作用域，不是调用时的作用域。</p>
<h2 id="4-参数"><a href="#4-参数" class="headerlink" title="4. 参数"></a>4. 参数</h2><h3 id="4-1-概述"><a href="#4-1-概述" class="headerlink" title="4.1 概述"></a>4.1 概述</h3><p>函数运行需要提供的外部数据。</p>
<h3 id="4-2-参数的省略"><a href="#4-2-参数的省略" class="headerlink" title="4.2 参数的省略"></a>4.2 参数的省略</h3><ul>
<li>JS允许省略参数。</li>
<li>函数length属性只反映预期传入参数个数。</li>
<li>要省略靠前的参数，显示传入undefined。<h3 id="4-3-传递方式"><a href="#4-3-传递方式" class="headerlink" title="4.3 传递方式"></a>4.3 传递方式</h3></li>
<li>传值传递（passes by value）传拷贝<br>参数是原始类型的值（数值、字符串、布尔值）。<br>函数体内部修改参数不影响外部。</li>
<li>传址传递（pass by reference）传地址<br>参数是复合类型的值（数组，对象，其他函数）。<br>函数内部修改会影响原始值。<h3 id="4-4-同名参数"><a href="#4-4-同名参数" class="headerlink" title="4.4 同名参数"></a>4.4 同名参数</h3>如果有同名参数，取后出现的那个值。<h3 id="4-5-arguements对象"><a href="#4-5-arguements对象" class="headerlink" title="4.5 arguements对象"></a>4.5 arguements对象</h3></li>
<li>arguments对象包含函数运行时的所有参数。</li>
<li>只有在函数体内部才能使用。</li>
<li>正常模式可修改，严格模式修改无效但不报错。</li>
<li>arguments.length 判断函数调用时到底带几个参数。</li>
<li>是伪数组，数组专有的方法不能在arguments对象上直接使用。</li>
<li>callee属性，返回它所对应的原函数。（这个属性严格模式禁用）<h2 id="5-函数其他知识点"><a href="#5-函数其他知识点" class="headerlink" title="5. 函数其他知识点"></a>5. 函数其他知识点</h2><h3 id="5-1-闭包"><a href="#5-1-闭包" class="headerlink" title="5.1 闭包"></a>5.1 闭包</h3>如果一个函数使用了它范围外面的变量，那么<strong>这个函数+这个变量</strong>就叫做闭包。<br>闭包：<a href="https://zhuanlan.zhihu.com/p/22486908" target="_blank" rel="noopener">https://zhuanlan.zhihu.com/p/22486908</a><h3 id="5-2-立即调用的函数表达式"><a href="#5-2-立即调用的函数表达式" class="headerlink" title="5.2 立即调用的函数表达式"></a>5.2 立即调用的函数表达式</h3><h2 id="6-eval命令"><a href="#6-eval命令" class="headerlink" title="6. eval命令"></a>6. eval命令</h2>eval 命令的作用是，将字符串当作语句执行。</li>
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